The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Known engine control strategies to improve fuel economy and reduce fuel consumption in internal combustion engines include operating at a lean air/fuel ratio. This includes control strategies for engines configured to operate in compression-ignition and lean-burn spark-ignition combustion modes. Engines operating at lean air/fuel ratios can have increased local combustion temperatures leading to increased NOx emissions.
A known exhaust aftertreatment system and control strategy for managing and reducing NOx emissions includes a urea injection control system and an associated ammonia-selective catalytic reduction device. The urea injection control system injects a reductant, e.g., urea into an exhaust gas feedstream upstream of the ammonia-selective catalytic reduction device. The injected urea decomposes to ammonia, which reacts with NOx in the presence of a catalyst to produce nitrogen. Some amount of ammonia can be stored on the ammonia-selective catalytic reduction device, enabling continued NOx reduction when the urea injection control system is not capable of dispensing a controlled amount of urea. Known control systems include dispensing urea at a rate that corresponds to concentrations of engine-out NOx emissions to achieve NOx reduction without using excess amounts of urea, i.e., at a urea/NOx stoichiometric ratio.